. The Na+,K+-ATPase is a heterodimeric enzyme that extrudes sodium from the cell in exchange for potassium at the expense of ATP catabolism. Physiologic functions of the Na+,K+-ATPase include regulation of cell volume, generation of ion gradients for transport of other solutes, maintenance of voltage gradients in excitable tissues, and pharmacologic inhibition by cardiac glycosides. Altered Na+,K+-ATPase activity has also postulated by several groups to be involved in either the regulation or the early response to induction of differentiation or activation of hematopoietic cells. We and others have recently demonstrated by molecular cloning that the Na+,K+-ATPases are encoded by a complex gene family specifying at least 3 alpha and 2 beta subunit isoforms. We have recently identified expression of a novel isoform, the A3 isoform, in leukemia cell lines of the monocyte/macrophage and lymphoid lineage. These cell types represent the only non neuromuscular cells in which expression of this isoform has been encountered in appreciable amounts. Moreover, expression of the alpha 3 mRNA can be modulated by induction of differentiation. We now propose to: (1) assess the distribution of alpha and beta isoform subunit expression in normal and leukemic blood cell lines, (2) investigate the mechanisms regulating the expression during differentiation or physiologic activation of these cells, and (3) explore the physiologic role, if any, played by the unexpected expression of the alpha 3 subunit in normal function of these cells. Toward these ends, we shall complete ongoing isolation and characterization of antibodies and DNA probes, permitting precise measurements of mRNA and protein levels of each isoform, isolate and characterize the promoters and enhancers regulating subunit expression in these cells, and devise systems whereby selective inhibition or enhancement of individual isoform production and/or activity can be achieved. The effect of the latter maneuvers upon induced differentiation and specialized function by the cell types will be assessed. In this manner, we hope to obtain new information about the physiologic importance of the novel A3 subunit, and the relationships between sodium transport and blood cell differentiation and function.